1. Field of the Invention
The present invention relates to an electrophotographic printer using a continuous recording sheet, and more particularly to an electrophotographic printer which is improved in printing accuracy and produces satisfactory print quality.
2. Description of the Related Art
The printing process in which an electrophotographic printer effects printing on a continuous recording sheet will be described. First of all, an electrostatic latent image is formed on a photosensitive drum by utilization of photoconductivity. Then, the electrostatic latent image on the photosensitive drum is developed and visualized with toner, to thereby form an toner image. The toner image, thus formed, is transferred onto a continuous recording sheet supplied to the location of the photosensitive drum. Thereafter, the toner image on the continuous recording sheet is fixed, so as to complete the printing.
As is apparent from the above, the electrophotographic printer is provided with a sheet-feeding device which feeds the continuous recording sheet to the photosensitive drum. The sheet-feeding device includes a tractor unit for feeding the continuous recording sheet to the photosensitive drum, and a pull unit for pulling the continuous recording sheet from the location of the photosensitive drum.
As is disclosed in Japanese Patent Publication No. 62-59303, a pull unit is placed on the downstream side of a photosensitive drum with reference to the direction in which the continuous recording sheet moves. The pull unit includes an endless belt traveling in one direction. This endless belt is extended in the sheet-conveying direction and has a large number of suction holes formed therein. A suction fan is arranged between the top and bottom turns of the endless belt, so as to produce air streams passing through the suction holes of the endless belt. Thus, the suction fan is worked under the movement of the endless belt, and the continuous recording sheet, which has passed by the photosensitive drum, is sucked onto the moving endless belt and is conveyed in accordance with the movement of the endless belt.
However, the suction-type pull unit mentioned above has problems, in that the sheet-pulling force varies depending upon the width of a continuous recording sheet to be used. If the continuous recording sheet is narrower than the endless belt, some of the suction holes of the endless belt are outside of the range of the continuous recording sheet and are therefore exposed. If the suction holes are partly exposed, air is sucked mainly through the exposed suction holes, with the result that the suction force is reduces. Therefore, the continuous recording sheet cannot be reliably pulled, and it cannot be conveyed in a stable manner.
As the continuous recording sheet, a fanfold sheet is often used which can be folded or separated along folding lines provided in the longitudinal direction at regular intervals. Assume that this fanfold sheet is used in the above-mentioned printer and is not sufficiently pulled during conveyance. In this case, it is likely that the fanfold sheet will not be stretched tight during conveyance, particularly at the portions located in the neighborhood of the folding lines. Since, therefore, the portions in the neighborhood of the folding lines may not be brought into reliable contact with the photosensitive drum, defective image transfer is likely to occur, resulting in so-called image omission. If the fanfold sheet is thick, the defective image transfer occurs very often, since the thick fanfold sheet tends to be folded or bent at the folding lines.
The pull unit mentioned above is a type which is made up of a suction fan and suction holes. In addition to this type, another type of pull unit is known in the art, which comprises a feed roll driven with constant torque, and a nip roll which is in rolling contact with the feed roll. A fanfold sheet is fed such that it passes through the region between the two rolls. However, this type of pull unit has a similar problem to that mentioned above. When the folding line portions of the fanfold sheet pass the region between the rolls, the nip roll is raised or moved away from the feed roll. As a result, the force with which to pull the fanfold sheet is temporarily varied. Consequently, the toner image on the photosensitive drum may not be transferred onto the intended position on the fanfold sheet.
In the electrophotographic printer, the device for feeding a continuous recording sheet requires improvement, so as to solve the above-noted problem and therefore to enhance the printing accuracy. Further, a cleaning device, which removes the toner (i.e., a developer) remaining on the photosensitive drum after the image transfer, a photographic fixing device, which fixes the toner image transferred onto the continuous recording sheet, and some other structural components also require improvement, so as to enhance the print quality.